We will test the hypothesis that protacyclin, PGI2, plays a significant role in the maintenance of the non-thrombogenicity of the vascular endothelium. PGI2 is synthesized by the endothelium and is released in response to several stimuli including injury and the active procoagulant, thrombin. The overall objectives of this project involve 1) the study of mechanisms that regulate PGI2 biosynthesis and release, 2) the study of mechanisms involved in adherence of platelets to vascular cells, 3) defining the role of prostacyclin in 2) and its limitations and 4) the study of mechanisms involved in thrombin-induced prostacyclin release and the refractoriness which follows the initial thrombin stimulus. The approach to the work will include studies on isolated platelets and endothelial cell preparations, cultured vascular cells, isolated vessel and organ preparations and normal human subjects. The mechanisms whereby prostaglandins influence the reactivity of platelets and endothelium will be examined using assays of specific prostaglandin derivatives in association with cellular biochemical changes and alterations in functional capabilities of the cell. The program will use a coordinated approach to provide new insight into control mechanisms which operate in the pathogenesis of hemorrhage and thrombosis.